An important aim of ongoing research in the semiconductor industry is increasing semiconductor performance while decreasing power consumption in semiconductor devices. Planar transistors, such as metal oxide semiconductor field effect transistors (MOSFETs) are particularly well suited for use in high-density integrated circuits. As the size of MOSFET and other devices decrease, the dimensions of source/drain regions, channel regions, and gate electrodes of the devices, also decrease.
The design of ever-smaller planar transistors with short channel lengths makes it necessary to provide very shallow source/drain junctions. Shallow junctions are necessary to avoid lateral diffusion of implanted dopants into the channel, since such diffusion disadvantageously contributes to leakage currents and poor breakdown performance. Shallow source/drain junctions, with a thickness on the order of 1000 Å or less, are generally required for acceptable performance in short channel devices.
Silicon-on-insulator (SOI) technology allows the formation of high-speed, shallow-junction devices. In addition, SOI devices improve performance by reducing parasitic junction capacitance.
In a SOI substrate, a buried oxide (BOX) film made of silicon oxide is formed on single crystal silicon, and a single crystal silicon thin film is formed thereon. Various methods for fabricating such SOI substrates are known. One such method is Separation-by-Implanted Oxygen (SIMOX), wherein oxygen is ion implanted into a single crystal silicon substrate to form a buried oxide (BOX) film.
Another method of forming a SOI substrate is wafer bonding, wherein two semiconductor substrates with silicon oxide surface layers are bonded together at the silicon oxide surfaces to form a BOX layer between the two semiconductor substrates.
Another SOI technique is Smart Cut®, which also involves bonding first and second semiconductor substrates through oxide layers. In the Smart Cut® method, the first semiconductor substrate is implanted with hydrogen ions prior to bonding. The hydrogen ion implanting subsequently allows the hydrogen ion implanted semiconductor substrate to be split from the bonded substrates leaving behind a thin layer of silicon bonded to the surface of the second semiconductor substrate.
Semiconductor device performance can be further enhanced by 50% or more by fabricating a P-type MOSFET (PMOSFET) on silicon with a <110> crystal orientation rather than a conventional <100> orientation. However, the performance of a N-type MOSFET (NMOSFET) formed on <110> silicon may be degraded compared to a NMOSFET formed on silicon with a <100> orientation.
Semiconductor device performance can also be enhanced by fabricating fully depleted MOSFETs on very thin silicon films, such as films with a thickness of about 30 nm or less. Fully depleted MOSFETs provide reduced current leakage and are desirable for high performance devices. However, it is difficult to modify the threshold voltage of fully depleted MOSFETs with conventional techniques, such as adjusting a halo dose. It is desirable to modify the threshold voltage of MOSFETs to create high and low threshold voltage devices. Semiconductor devices comprising MOSFETs with a range of different threshold voltages reduces the power consumption of the chip.
The term semiconductor devices, as used herein, is not to be limited to the specifically disclosed embodiments. Semiconductor devices, as used herein, include a wide variety of electronic devices including flip chips, flip chip/package assemblies, transistors, capacitors, microprocessors, random access memories, etc. In general, semiconductor devices refer to any electrical device comprising semiconductors.